In the medical field ionizing radiation has many diagnostic and therapeutic uses. It can be used for imaging the body of a patient for example to detect tumors and it can also be used for the treatment of a patient for example to irradiate detected tumors. In order to obtain good resolution images and accurately placed treatment it is necessary to detect the ionizing radiation after it has passed through the body being examined or treated and then process the signals obtained for the detecting means. The signals can be processed to obtain an image which represents the body through which it has passed and/or which represents the distribution of the ionizing radiation. In general the term “imaging” means forming an image of the measured data, and the term “dosimetry” measurement of the absorbed dose due to incident ionizing radiation.
In order to detect in real time the intensity and spatial distribution of incoming ionizing radiation such as high energy photons generated in X-ray imaging or treatment devices, digital detecting units have been developed which convert the incoming radiation, for example photons, into electrons. This can be achieved by electromagnetic interaction with the photons and matter which can cause some of the energy of the photon to be transferred to the matter and a free electron to be excited. Devices within a detecting unit which achieve this effect are called converters. Solid converters rely on the interaction of the incident photons with a solid sheet of material to generate electrons by the Compton Effect or pair production. Gas converters rely on the interaction of incident photons with a gas in a chamber to generate electrons. An example of a prior art unit is known from PCT patent application WO2007/061235. A simplified example of a digital detecting unit (1) is shown in FIGS. 1a) to 1d). This device comprises a housing (3) (shown in dashed lines) filled with a gas (16). The housing contains a converter (5), a GEM (7) separated from the converter by a gas filled gap of depth G, and a read-out unit (9). The converter is formed from a substrate of metal (11). The substrate has a first major face (13) which faces incident ionizing radiation I. This incident radiation causes the formation of electrons in the substrate and these electrons travel through the substrate towards the second major face (15) of the substrate. Electrons which pass through the substrate and impact with the gas molecules in the gas-filled gap cause ionization of the gas molecules and the production of secondary electrons. These electrons need to be multiplied in order to achieve a detectable signal. One way of performing electron multiplication is by providing a micropattern gas amplification device (MPGAD) such as a gas-electron multiplier (GEM) (7) of the type developed by CERN, separated from the second major face of the converter by the gas-filled gap of depth G. The GEM comprises a thin, insulating, perforated gas electron multiplication foil (17). Typically the foil is made of an insulating substrate (19) of polyimide polymer poly-oxydiphenylene-pyromellitimide, usually called “Kapton®”, that is coated on both major sides (21, 23) with a coating (25, 27) made of a conducting material such as copper. The perforations in the foil form a regular matrix (29) of GEM through holes (31) which extend between the major sides (21, 23) and which form a grid of equidistantly-spaced GEM through holes over substantially the whole of the major surfaces. A potential difference can be placed across the coatings (25, 27) thereby generating an electric field in the GEM through holes which electrical field guides electrons from the converter into the GEM through holes (19). The electrical field generated m the gas in the through holes initiates electron avalanches which increase the number of electrons leaving each through hole. The number of electrons generated in the avalanche can be in the range of 100-1000 per incoming electron. These electrons can be collected and the position and intensity of the incident radiation determined by processing signals generated in the read-out unit which could be, for example a thin film transistor. The electrical fields necessary to ensure the electrons are guided to the read-out unit can be achieved by connecting the converter to a high negative potential, for example −700 V, the upper surface of the GEM to a medium negative potential, for example −600 V, the lower surface of the GEM to a lower negative potential e.g. −300 V and the read-out unit to ground. In order to achieve a large amplification of the electrons leaving the converter it is necessary to have a large potential difference between the converter and the GEM. However, if the potential difference is too high then there is a risk of electrical discharges in the form of sparking between the converter and the GEM.
A problem with the prior art devices is that high energy electrons which leave the electron-emitting face of the converter with a large lateral vector will collide with many gas molecules as they travel laterally in the gas gap between the converter and the GEM. These gas molecules will generate electrons, some of which will travel to the GEM and be multiplied by the GEM in a through hole which is not directly under the arrive point of the incident ionizing radiation on the first major face of the substrate which gave rise to the high energy electron and thus will an erroneous position of the incident ionizing radiation when they are detected by the read-out unit. These give rise to errors in the signals produced by the rad-out unit and lead to decreased accuracy and resolution in images of the incident radiation
Another problem which can occur is that a large GEM acts as a capacitor which leads to unwanted electrical fields and the possibility of dangerous charges becoming stored in the device.